Relationships between the abundance of bacteria and other biota and the hydrographic variability in the Ría de Vigo, Spain

Abstract

Hydrographic variability driven by changes in wind speed and direction and by meteorological parameters, such as incoming solar radiation, fluvial discharges, and temperature, affects biological standing stocks (e.g. of bacteria) in the marine environment. We studied the effects of such parameters in the Ría de Vigo (Galicia, Spain), between October 1990 and August 1991, where downwelling in winter results from predominantly poleward winds, whilst in summer upwelling is driven by winds travelling towards the equator. Total bacterial abundance was strongly affected by physical factors in the Ría de Vigo. Almost 60% of the variance in bacterial abundance in the surface layer could be explained by an equation relating logTC [log(total count of heterotrophic bacterioplankton)] to incoming solar radiation, water temperature, run-off and coastal upwelling, where solar radiation, water temperature and moderate upwelling are all positive terms, while run-off and strong upwelling act negatively. Consideration of biological variables such chlorophyll, ciliates and heterotrophic flagellates did not further explain the variance. Physical factors, however, only accounted for 40% of the total variance in bacterial abundance at the bottom of the photic layer. The inclusion of biological variables in the regression equations increased the explained variance to 55%; chlorophyll was the most important of these variables and was positively related to logTC. This indicates that bacterial populations in the surface layer are mainly controlled by physical factors with a weak phytoplankton influence; however, a stronger coupling between primary production and bacterial growth must exist at the bottom of the photic layer. Physical variables also explained 60 to 65% of the variance in cultivable spore-forming saprophytic bacteria, but, in contrast to effects on total bacterial numbers, run-off had a positive effect while moderate upwelling had a negative influence on spore-forming numbers. This suggests that terrestrial run-off has some ecological importance through the enrichment of nearshore bacterial populations with cells of terrestrial origin.